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Sergei Medvedev, University of Oslo (Norway)
Timm John, University of Oslo (Norway)
Torgeir Andersen, University of Oslo (Norway)
Yuri Podladchikov, University of Oslo (Norway)
Håkon Austrheim, University of Oslo (Norway)
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Convergent margins are characterized by strong seismic activity with earthquakes occurring at depths of up to 700km. Shallow earthquakes (<60km) are explainable by the brittle failure of rocks. At greater depths, the increased ambient pressure should inhibit brittle failure. We present a self-localising thermal runaway (SLTR) as a possible mechanism for intermediate to deep earthquakes. Recently discovered, this phenomenon presents an ultimate failure mechanism for viscoelastic materials. Irreversible ductile deformation dissipates mechanical work into heat that can lead to thermal softening and failure by progressively self-localizing deformation, SLTR. This failure mechanism, in contrast to brittle failure, is independent from ambient pressure. We present the results of an analytical, numerical and petrological study that evaluates SLTR as a failure mechanism for intermediate to deep earthquakes. Our numerical experiments compare well to field evidence from the Krakenes Gabbro in Western Norway. Particularly, our experiments show possible coexistence of ductile (shear zones) and brittle-like (pseudotachylyte veins) deformation and catastrophic failure with seismogenic strain rates and active melting in the deep-earth conditions. We show that weakening by hydration also plays a key role in shear zone formation and ultimately seismic failure. We also compare differential stresses required for SLTR and compare them to brittle yield stresses according to Byerlee's law. This study shows that at depths greater than 60-80 km failure by SLTR is more likely than brittle failure, and that SLTR is a viable mechanism for earthquakes in subducting slabs and continental root zones.
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